The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2001-266460 filed on Sep. 3, 2001, which is incorporated herein by reference in its entirety.
The present invention belongs to a technology to record the information onto the optical disk by using the laser light beam.
The laser light is irradiated onto the recording surface of the disk and the information is recorded in the writable or re-writable optical disk such as DVD-R (DVD-Recordable) or DVD-RW (DVD-Re-recordable). At a portion onto which the laser light is irradiated, on the recording surface of the optical disk, because the temperature rises, a change is generated in the optical recording medium constituting the optical disk, thereby, a recording mark is formed on the recording surface.
Accordingly, by the recording pulse having the time width corresponding to the information to be recorded, the laser light is modulated, and the laser pulse having the length corresponding to the signal to be recorded is generated, and by irradiating this onto the optical disk, the recording mark having the length corresponding to the information to be recorded, can be formed on the optical disk.
On the one hand, recently, one recording mark is not formed by the one laser pulse, and a method by which the recording mark is formed by the pulse train including a plurality of short pulses, is used. Such a method is also called write strategy, and as compared to a method by which a single recording pulse is irradiated, because the heat accumulation on the recording surface of the optical disk is reduced, the temperature distribution on the recording surface on which the recording mark is formed can be uniformed. As the result, it is prevented that the recording mark becomes the tear drop shape, and the preferably shaped recording mark can be formed.
The above recording pulse train is structured by a plurality of pulses whose amplitude is varied between a predetermined read power level and write power level. That is, according to the recording signal, in the area on the recording surface of the optical disk (hereinafter, called also “space portion”) in which the recording mark is not formed, the laser light is irradiated onto the recording surface by the read power, and in the area on the recording surface of the optical disk (hereinafter, called also “mark portion”) in which the recording mark is to be formed, the laser light is irradiated onto the recording surface by the power corresponding to the recording pulse train whose amplitude is varied between the read power and the write power, thereby, the recording mark is formed on the recording surface.
However, when the laser pulse corresponding to the recording pulse is irradiated onto the recording surface of the optical disk, by the transient response of the pulse, the laser irradiation level (bias level) after the irradiation of the recording pulse is increased. The recording pulse whose amplitude is varied between the read power and write power, and after the completion of the recording pulse, theoretically, the laser irradiation level instantaneously returns to the read power. However, practically, at the time of completion of the recording pulse, because the laser irradiation level instantaneously changes from the write power to the read power, the transient response of the recording pulse is generated, and the laser irradiation level does not instantaneously return to the read power, but rather than it, the laser irradiation level is increased for a constant period. As the result, also after the recording pulse is completed, although it is a short time, a period in which the laser irradiation level is higher than the read power level, is generated. There is a case where this influences the formation of the next recording mark as the heat interference. Particularly, when the space period to the next recording mark is short, because there is the residual heat by the above transient response at the time of formation of the next recording mark, there is a case where the next recording mark can not be correctly formed.
Then, this influence is specifically prominent when the recording speed is increased, that is, when the high speed recording is conducted onto the optical disk. In the case of the recording of the normal speed (low speed), because a time period to the mark period corresponding to the next recording mark can be secured to some extent, the influence of the above transient response is softened, however, when the recording speed is increased to 2 times or 3 times of the normal one, because the time interval of the recording pulse train becomes short, the next recording pulse train comes under the condition that the residual heat is remained, and the possibility that the residual heat due to the above transient response influences the next recording mark as the heat interference is increased.
Further, when the recording speed is increased, in order to form the recording mark correctly, it is necessary that the recording power is increased corresponding to the increased amount of the recording speed. Accordingly, the increase of the laser irradiation level becomes large, and the bad influence is increased.
Then, because the increase of the laser irradiation level due to the transient response of the recording pulse is generated after the application of the recording pulse as described above, that is, in the space period of the recording signal, this influence can not be removed by the write strategy technology by which the pulse width of the recording pulse is adjusted, and the recording mark shape is adjusted.